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1.1 - Leaf anatomy, light interception and gas exchange

 

Leaves experience a mix of demands under frequently adverse conditions. They must intercept sunlight and facilitate the uptake of CO2, which exists at levels around 390 ppm (µL L-1) in the atmosphere, while restricting water loss. The wide variety of shapes, sizes and internal structures of leaves imply that many solutions exist to meet these mixed demands.

In nature, photon irradiance (photon flux density) can fluctuate over three orders of magnitude and these changes can be rapid. However, plants have evolved with photosynthetic systems that operate most efficiently at low light. Such efficiency confers an obvious selective advantage under light limitation, but predisposes leaves to photodamage under strong light. How then can leaves cope? First, some tolerance is achieved by distributing light over a large population of chloroplasts held in architectural arrays within mesophyll tissues. Second, each chloroplast can operate as a seemingly independent entity with respect to photochemistry and biochemistry and can vary allocation of resources between photon capture and capacity for CO2 assimilation in response to light climate. Such features confer great flexibility across a wide range of light environments where plants occur and are discussed in Chapter 12.

Photon absorption is astonishingly fast (single events lasting 10–15 s). Subsequent energy transduction into NADPH and ATP is relatively ‘slow’ (10–4 s), and is followed by CO2 fixation via Rubisco at a sedate pace of 3.5 events per second per active site. Distributing light absorption between many chloroplasts equalises effort over a huge population of these organelles, but also reduces diffusion limitations by spreading chloroplasts over a large mesophyll cell surface area within a given leaf area.  The internal structure of leaves (shown in the follwing section) reflects this need to maximise CO2 exchange between intercellular airspace and chloroplasts and to distribute light more uniformly with depth than would occur in a homogeneous solution of chlorophyll.